Hickory: In my opinion there is no point dual classing in BG1, particularly in your case because you will not recover your fighter class until the game is virtually finished. Added to that, mages get very few spell slots in BG1 (in vanilla, anyway), and the lack of kits means you will be gimped. That doesn't mean the game will become unwinnable, just that I don't see much point in it. You would have been better off multi classing in BG1.
Edit: I notice you state 'Single Weapon Style'. That means you're not playing vanilla, so it might help to state exactly what mods you are running.
novanus: It's been quite a while but I'm fairly sure I followed the guide posted in "STICKY: Newbie guide; modding BG1 if you buy BG1 + BG2 from GOG". I didn't install Unfinished Business though.
Anyway, thanks for the reply. Yeah that is kinda what I was worried about. I guess I will stick it out with fighter. Thing is though, I had planned to transfer my party to BG2 when I start that, and from the little I have read it seems magic is important in that one, so a pure fighter may be weak? When I start BG2 if I use my existing save will I have an opportunity to redo the build?
How are you launching the game? Are you running it by loading the baldur's gate 2 exe? If so, you're playing with Baldur's Gate Trilogy, which will automatically combine the two games into one. If you're launching it through some other method, then you're probably following the guide you mentioned, and have Tutu installed. Either way, it sounds like you're playing with the BG2 engine.
I'm going to assume that you're at least fairly familiar with how dual classing works: to remind you, you choose to dual class at any time, though generally immediately after levelling, and then you lose the class abilities bar hp of your first class (fighter) and begin levelling as the second class (mage). When you have more mage levels than fighter levels, you'll regain your fighter abilities and be rather powerful.
I'm afraid I haven't done a fighter/mage dual class. I have, however, done a fighter/cleric, which I'd imagine has similarities during the early part of the dual classing. I've also done a thief/mage, which would be similar later on. To begin with, your character will receive something of a nerf: although you retain the hit points so are still quite survivable, you're going to lose a lot of thac0 (hit chance, if you've forgotten) and other things. For a while, you'll be virtually useless, casting your couple of first level spells and then shooting at enemies with your bow, and hoping to hit. During this time, your party will be covering for you: in combat, you'll effectively have about 5 and a half party members. However, because the xp costs to level grow exponentially (as do the xp rewards for killing stronger monsters), you'll gain your first 6 or so levels quite quickly, and will then be a useful if somewhat weaker than normal character.
When you regain your fighter abilities, you'll obviously still be a bit behind Dynaheir in terms of levels, but will have much more survivability and will be able to actually fight in combat, which is more than can be said for a pure mage. What's more, as time passes (particularly in BG2) due to the aforementioned exponential growth, this will become more and more inconsequential quite rapidly. Within a level or so after you regain your fighter abilities, you'll overtake a pure mage like Dynaheir, and probably become one of the more - if not the most - useful character in your party.
As for when to do it, that depends somewhat on what you plan to use your fighter/mage for. If you just want a mage with some more hp so they don't die so rapidly, then you should dual class now, and should probably have done so some time ago, at level 2 or 3. However, fighter/mages shine especially well with buff spells like (in BG2) stoneskin and tenser's transformation. The latter in particular is intended to transform a mage into someone on about a level with a fighter for melee combat. Guess what happens when you get a fighter/mage to cast it. If you plan to use buffing spells and fight - sometimes - in melee or with a bow, then you should get that level 7 for the half attack. I'm not sure what xp cap Tutu has in place, but you'll still be able to get your fighter abilities back within the TotSC xp cap. I'd recommend going down that route, especially if you want to try out spells to let you fight in melee.
Either way, with those stats, you'll make a significantly better mage than fighter, since Con is...not great for a tank, while you have 18 Intelligence.
You mentioned that you weren't concerned with being min/maxed to perfection, so I'm going to disagree with Hickory here and say that you should dual class during BG1, when you hit seventh level. The dual class may be slower, but you'll have come into your own as a fighter/mage in plenty of time for the last battles of BG1. I think you'll also have a more in depth and - hopefully - enjoyable experience of the dual classing process if you aren't exploiting the rather cheesy tactics that Hickory mentioned (wait for the BG2 xp rise, then immediately start dualling). If you do decide to go down that route, though, you'll probably be best off dualling at either level 10 - when you stop gaining large amounts of hp - or level 13, when you get another half attack. I'll also disagree with him about choosing Kensai. The Kensai/mage dual class is powerful - extremely powerful, more so than any other combination of classes. The reasons should be fairly obvious: the Kensai is a fighter which focusses solely on weapons and can't wear armour, and you can't cast spells in armour (with a few exceptions to be found in BG2: keep your eye out for them). Again, you're not trying to min/max, so you may well find the combination too powerful and find you eclipse the rest of your party to some extent. I'd suggest you just stick with the standard fighter, unless you want that kind of power.
Incidentally, I don't know if you did this intentionally or if it was just chance, but you happen to be playing with the so called "canon party": congratulations! BG2 behaves as if you played BG1 with those party members, so the beginning of BG2 will be consistent with your experiences in BG1, and won't hurt your immersion. In BG2, Imoen has dual classed to a mage at 7th level or so. If you want to keep her in the party, that's another reason that you may want to have a slightly different focus with your own fighter/mage. For this game, feel free to dual her or not as you see fit: she makes quite a good mage, but the dualling comes at an inconvenient time in my experience - just when thief skills become vital - so you may prefer to wait until BG2 and assume she was learning the secrets of magic behind the scenes from Dynaheir, and unlocked the abilities during the interim between BG1 and 2! Either way, unless you're playing with BGT (in which case, for plot purposes, you probably should dual class Imoen: her ability to use magic is relevent to the storyline), her statistics will be reset to how they start at the beginning of BG2 when you start: only your own character's stats are carried over.
EDIT: I think the message you're being given by us between us is: whether you dual or not, you'll still be pretty powerful, so just do whichever you prefer. If you do dual, though, do so at level 7, 10 or 13, and switch to kensai at BG2's start if you want more a cheesy way to get more power.
